A more recent report I read indicated that the "explosive weight" is a conversion from the actual weight of the explosives to TNT equivalent, so it isn't as simple as setting the fireworks on a bathroom scale and seeing what comes out. They had to estimate the weight of the flash powder inside the fireworks and then convert it.
They also said they estimated weight instead of weighing each one to reduce handling of the devices.
In any case, they messed up but good.